1. Field of the Invention
The present invention relates to the technical field of laminate-type batteries such as coiled electrode batteries, and of manufacturing processes for electrodes that can be employed in such laminate-type batteries.
2. Description of the Prior Art
As lithium ion secondary batteries with high energy density per unit volume and per unit weight have become commonly used as power sources for cellular telephones, portable video cameras, portable data terminals and the like, attention is also being focused on their use as batteries for electric automobiles.
When lithium ion secondary batteries are used for electric automobiles, however, the requirement to provide high power has presented problems of heat generation and insufficient output because of the high internal resistance of the batteries in the commonly employed system of current output through tabs. That is, it has been difficult and thus inconvenient to achieve high output by current collection from coiled electrodes with simple tabs. However, when it is attempted to weld multiple tabs to coiled electrodes, the multiple tabs become welded to the coiled electrodes during coiling, resulting in the inconvenience of low productivity as the number of steps in the coiling process is increased.
With the aim of eliminating this inconvenience, Japanese Unexamined Patent Publication No. 55-80269 has disclosed a coiled electrode battery which is based on the prior art.
This coiled electrode battery has a coiled formed by electrode coiling both a belt-like positive plate and negative plate having protrusions protruding out along the axial length of the coil in opposite directions to each other, and a separator lying between the positive plate and negative plate. That is, both the positive plate and negative plate have a protrusion protruding out from the edges of each electrode plate of with opposite polarity (opposite polarity plates). The positive plate and negative plate have respective collectors welded thereto, and the positive plate and negative plate are connected through these collectors to a pair of positive/negative electrode terminals that supply external power.
Thus, the coiled electrode batteries of the prior art require no connection of multiple tabs and can therefore be produced without sacrificing productivity, while their collectors with low electrical resistance allow them to exhibit large output without the risk of overheating.
Nevertheless, with the aforementioned coiled electrode battery of the prior art, it has been difficult to achieve production at high yields because the protrusions contact with the edges of the opposite polarity plates due to misaligned winding of the separator, producing short-circuits. For example, if the positions of the positive plate, negative plate and separator do not precisely match in the axial length direction of the coil, the opposite edge which is in the opposite direction from the protrusions will contact the adjacent protrusions of opposite polarity, causing a short-circuit. Since short-circuits can be easily caused by contact between the opposite edge of the negative plate and the protrusion of the positive plate when the separator is misaligned, the defect rate has often been significantly high in the production process.
This sort of problem is not limited to coiled electrode batteries but can also occur with other laminate-type batteries of the prior art, for example, batteries equipped with laminated electrodes prepared by forming disk-shaped positive plates and negative plates laid together with a separator between them, such as coin-type batteries, and batteries equipped with laminated electrodes formed by alternately laminating square-shaped positive plates and negative plates with separators between them, such as square batteries.
That is, even in these laminate-type batteries, if the alignment positions of the positive plate, negative plate and separator do not precisely match, the opposite edge which is in the opposite direction from the protrusion can contact the adjacent protrusion of opposite polarity, causing a short-circuit. As a result, since short-circuiting can easily occur, for example, if the opposite edge of the negative plate exposed by misalignment of the separator contacts with the protrusion of the positive plate, similar to the coiled electrode battery mentioned above, the reject rate has often been significantly high in the production process. Consequently, it cannot be said that laminate-type batteries of the prior art offer sufficiently high product yields.
On the other hand, all batteries of the prior art are susceptible to shrinkage of the separator upon abnormal increase of temperature in the battery during use. The possibility also exists that vibrations, etc. will cause the separator to shift from the prescribed position between the positive plate and negative plate. In these cases as well, the protrusions of the positive plate and negative plate which protrude out beyond the edges of the opposite polarity plates can produce short-circuits by contacting with the edges of the opposite polarity plates.
It is therefore an object of the invention to provide a laminate-type battery that can reliably prevent short-circuiting inside the battery and which allows higher product yields.
It is another object of the invention to provide an electrode-manufacturing process that can be used for such a laminate-type battery and that allows easy manufacture of electrodes.
The present inventors have invented the following means in order to achieve these objects.
(First aspect)
The first aspect of the invention is a laminate-type battery with a laminated electrode prepared by laminating a sheet-like positive plate and negative plate and a separator lying between the positive plate and the negative plate, characterized in that at least either one of the positive plate and the negative plate has a protrusion that protrudes out beyond the edge of the other electrode plate of opposite polarity (opposite polarity plate), and the protrusion has a short-circuit preventing layer on the surface of at least its proximal section. The protrusion may be a conductor formed separately from the electrode and bonded to the electrode plate, or it may be formed integrally with the electrode. It is particularly preferred for the protrusion to protrude from the edge of the opposite polarity plate which is adjacent to it via the separator.
According to this aspect, the laminate-type battery which allows contact between the electrode plate with the protrusion and the non-protruding edge (non-protruding section) of the opposite polarity plate, is provided with a short-circuit preventing layer at the proximal section of the protrusion. Consequently, even if the non-protruding section contacts with the proximal section of the protrusion due to lamination misalignment of the separator when the positive plate and negative plate are laminated, the presence of the short-circuit preventing layer lying between the non-protruding section and the proximal section of the protrusion prevents short-circuiting between the positive and negative poles. In addition, when there is a risk of the non-protruding section contacting with the protrusion due to burrs on the non-protruding section, short-circuiting between the positive and negative poles is prevented in the same manner by the short-circuit preventing layer situated at the proximal section of the protrusion.
According to this aspect, therefore, short-circuiting between the positive and negative poles in the battery can be reliably prevented for an effect which allows laminate-type batteries to be provided at higher product yields.
With this aspect, the external leads of the device to which the power from the battery is to be supplied can be directly connected to the positive plate and negative plate of the laminate-type battery, but for the purpose of increasing the power current-collecting efficiency and sealing of the electrolyte solution, it is preferred to provide a pair of positive and negative electrode terminals which are connected to the positive plate and negative plate, respectively, and supply power externally. In this case, it is preferred for at least either one of the positive plate and negative plate to have protrusions that protrude out beyond the edge of the opposite polarity plate and are connected to the electrode terminal, and for each protrusion to have a short-circuit preventing layer on the surface of its proximal section.
In a laminate-type battery where such protrusions are connected to the electrode terminals it is not possible for the edge of the separators to protrude out beyond the protrusions. In a battery where the protrusions must necessarily protrude beyond the edge of the separators, such as a laminate-type battery in which the protrusions are connected to the electrode terminals in this manner, slight lamination misalignment of the separator may create contact between the protrusion and the edge of the opposite polarity plate from the electrode plate with the protrusion. It is highly expensive to prevent such slight laminate misalignment of the separator.
This aspect is particularly effective for batteries wherein the protrusion protrudes out not only beyond the edge of the opposite polarity plate but also beyond the edge of the separator lying between the electrode plate with the protrusion and the opposite polarity plate. In other words, the above-mentioned short-circuit preventing layer can reliably prevent short-circuits between the positive and negative poles even when the separator experiences slight lamination misalignment. The short-circuit preventing layer can be inexpensively and easily situated, compared to efforts to prevent slight lamination misalignment of the separator. The resulting effect can economically provide laminate-type batteries with higher product yields.
Particularly when the protrusions are connected to the electrode terminals, the protrusions of the electrode plates sometimes bend in the direction of the electrode terminals, thus deforming the protrusions. When this occurs, the deformed protrusions can easily contact with the edge of the opposite polarity plate, and slight lamination misalignment of the separator can risk creating contact between the protrusions and the edges of the opposite polarity plates. It is highly expensive to prevent such slight lamination misalignment of the separator. According to this aspect, the short-circuit preventing layer described above can reliably and inexpensively prevent short-circuits between the positive and negative poles even when the protrusions contact the electrode terminals in this manner, and the resulting effect can economically provide laminate-type batteries with higher product yields.
This aspect can be applied to laminate-type batteries having publicly known laminated structures, such as batteries equipped with laminated electrodes prepared by forming disk-shaped positive plates and negative plates laid together with a separator between them, such as coin-type batteries, and batteries equipped with laminated electrodes formed by alternately laminating square-shaped positive plates and negative plates with separators between them, such as square batteries. There is also no particular limitation to these types of batteries, and it may be applied to publicly known types of laminate-type batteries such as lithium batteries. It can also be applied to both primary batteries and secondary batteries.
For example, when this aspect is applied to a lithium secondary battery, the following materials may be used for the positive plate, negative plate, separator and electrolyte solution.
A publicly known positive electrode active material may be used as the active material for the positive plate. Among them it is preferred to use complex compounds of lithium with transition metals, such as lithium manganate (LiMn2O4), lithium cobaltate, lithium nickelate, etc. These complex compounds exhibit excellent discharge properties of lithium ions during charge (deintercalation performance) and occlusion properties of lithium ions during discharge (intercalation performance), and are therefore excellent for the charge/discharge reaction of the positive plate.
A publicly known carbon material can be used as the active material for the negative plate. Preferred for use are those comprising highly crystalline natural graphite and artificial graphite. Such highly crystalline carbon materials exhibit excellent intercalation performance of lithium ions during charge and deintercalation performance of lithium ions during discharge, and are therefore excellent for the charge/discharge reaction of the negative plate. Oxides and sulfides can also be used as active materials in addition to carbon materials.
Both the positive plate and negative plate are preferably electrode plates having a layer containing an electrode active material formed on a collector plate with excellent electric conductivity. In such cases, the protrusion may be formed integrally with the collector plate.
The separator used may also be made of a publicly known material, and for example, separators comprising polyolefin-based polymers such as polyethylene and polypropylene may be used.
The electrolyte solution used may be a publicly known one. Particularly preferred for use are non-aqueous electrolyte solutions prepared by dissolving lithium salts such as LiPF6 in organic solvents such as ethylene carbonate.
(Second aspect)
The second aspect of the invention is characterized by being similar to the first aspect but having a coiled electrode construction wherein the positive plate, negative plate and separator are coiled while laminated together, and the protrusion protrudes out in the coiled axial length direction.
Since a laminated electrode that is a coiled electrode experiences a dynamic state wherein the coiled electrode rotates while it is wound up during formation, it is prone to winding misalignment of the separator. Once a winding misalignment of the separator occurs, however slight, during formation of the coiled electrode, the winding misalignment will often become larger as coiling continues. Rectification of winding misalignments of the separator during coiling is not only difficult, but when the winding separator is forced in the opposite direction of the winding misalignment to prevent the winding misalignment from increasing, abnormal stress is generated not only on the separator but also on the electrode plates, which can result in failure to form a coiled electrode of satisfactory quality.
According to this aspect, the presence of the short-circuit preventing layer lying between the non-protruding section and the proximal section of the protrusion reliably prevents short-circuiting between the positive and negative poles even when winding misalignment of the separator occurs. It is therefore possible to easily obtain coiled electrodes with satisfactory quality guaranteed even when separator misalignments occur.
According to this aspect as well, as described for the first aspect, it is preferred to provide a pair of positive and negative electrode terminals which supply power externally, and at least a part of the coil axis core (core rod) may serve as the electrode terminal. In this case, the protrusion may be connected to the electrode terminal and it is preferred for the protrusion to have the short-circuit preventing layer situated on at least the proximal section on the inner surface, of the inner and outer surfaces.
According to this aspect, the short-circuit preventing layer is situated at the proximal section of at least the inner surface, of both surfaces of the protrusion with which the non-protruding section can contact. Thus, when the protrusions are bent in the centripetal direction and connected to the electrode terminals, the presence of the short-circuit preventing layer lying between the non-protruding section and the proximal section of the protrusion prevents short-circuiting between the positive and negative plates even when the non-protruding section contacts with the inner surface of the proximal section of the protrusion. In addition, when there is a risk of the non-protruding section contacting with the outer surface of the protrusion due to burrs on the non-protruding section, short-circuiting between the positive and negative poles is prevented in the same manner by the short-circuit preventing layer situated at the proximal section of both surfaces of the protrusion. Thus, short-circuiting does not occur between the positive and negative poles even when misalignment of the separator occurs in the direction of the coiled axis length.
According to this aspect, therefore, short-circuiting between the positive and negative poles in the battery can be reliably prevented for an effect which allows coiled electrode batteries to be provided at higher product yields.
(Third aspect)
The third aspect of the invention is characterized in that in either the first aspect or the second aspect mentioned above, the short-circuit preventing layer is made of an insulator. That is, the short-circuit preventing layer is an insulating layer formed on the surface of the proximal section of the protrusion. The insulating layer can be formed by coating the surface of the proximal section of the protrusion with a resin that is insoluble in the electrolyte solution by a hot-melt method or solvent cast method, attaching tape made of an insulating material, or forming an electrically insulating oxide film. Formation of the short-circuit preventing layer with an insulating layer can more reliably prevent short-circuits.
According to this aspect, therefore, the effects of the first and second aspects can be further reinforced.
(Fourth aspect)
The fourth aspect of the invention is characterized in that in any one of the aforementioned first to third aspects, the short-circuit preventing layer is formed on the edge of the separator protruding up to the proximal section of the protrusion. Thus, since there are no additional constituent elements of the laminated electrode with respect to the first to third aspects, it is possible to manufacture a laminate-type battery more economically while preventing short-circuits between the positive and negative poles in the battery. This is particularly effective for coiled electrode batteries. That is, since there are no additional constituent elements for the coiled electrodes, coiled electrode batteries can be manufactured more economically while preventing short-circuits between the positive and negative poles in the battery.
According to this aspect, therefore, an effect is provided whereby coiled electrode batteries can be manufactured relatively inexpensively, in addition to the effects of the first to third aspects.
(Fifth aspect)
The fifth aspect of the invention is characterized in that in the aforementioned fourth aspect, the other electrode plate, as a second electrode plate adjacent to the one end of the electrode plates, as a first electrode plate, has a protrusion that protrudes out beyond the non-protruding edge of the first electrode plate, the separator has a shape which covers both sides of the first electrode plate and the non-protruding edge of the first electrode plate, and the section of the separator which covers the non-protruding edge isolates the non-protruding edge from the proximal section of the protrusion of the second electrode plate adjacent to the non-protruding edge.
According to this aspect, not only does the separator protrude up to the proximal section of the protrusion of the other electrode plate, but it has a shape with covers both sides and the non-protruding section of the first electrode plate. Consequently, because of the construction of the separator which covers both sides of one pole, it is sufficient to simply laminate the first electrode plate covered with the separator on both sides and the other electrode plate instead of providing two separators between the positive and negative poles, and the lamination step is therefore simplified. This is particularly effective for coiled electrode batteries. That is, since it is sufficient to simply coil the first electrode plate covered with the separator on both sides and the other electrode plate during the coiling step for the electrode plates, the lamination step is therefore simplified.
According to this aspect, in addition to the construction where the edge of the separator protrudes up to the proximal section of the protrusion of the first electrode plate to form the short-circuit preventing layer, the separator is also constructed so as to cover the non-protruding section of the first electrode plate, such that the section of the separator covering the non-protruding section isolates the non-protruding section from the proximal section of the protrusion of the other electrode plate adjacent to the non-protruding section, and therefore insulation can be more reliably achieved between the non-protruding section of the first electrode plate and the proximal section of the protrusion of the other electrode plate. This is also particularly effective for coiled electrode batteries.
According to this aspect, therefore, an effect is provided whereby the number of lamination steps is reduced and the insulation properties can be more reliably guaranteed, in addition to the effect of the fourth aspect.
(Sixth aspect)
The sixth aspect of the invention is characterized in that, in any one of the aforementioned first to fifth aspects, the separator is a porous film formed integrally with the surface of either or both of the protrusion-provided electrode plates of the positive plate and negative plate. The porous film is a film with large pores that allows electrolytes (electrolyte ions) to pass through.
According to this aspect, the porous film can fulfill the role of the separator. Here, it is less expensive to form a separator comprising a porous film integrated with the surface of the electrode plate, than to fabricate a normal film separator to form the laminated electrode. In addition, since it is not necessary to interlay two film separators between the positive plate and negative plate in the lamination step for the electrode plates, it is possible to further simplify the lamination steps and reduce the number of lamination steps. Furthermore, because it is sufficient to achieve a perfectly matched combination between only the positive plate and negative plate, reduced yields due to lamination misalignment of the separator can be avoided. Laminate-type batteries can therefore be manufactured with excellent productivity.
This is particularly effective for coiled electrode batteries. That is, it is less expensive to form a separator comprising a porous film integrated with the surface of the electrode plate, than to fabricate a normal film separator to assemble the laminated electrode. In addition, since it is not necessary to coil up two film separators between the positive plate and negative plate in the coiling step for the coiled electrode, it is possible to further simplify the coiling steps and reduce the number of coiling steps. Furthermore, reduced yields due to coil shifting of the separator can be avoided. As a result, coiled electrode batteries can be manufactured with excellent productivity.
According to this aspect, therefore, an effect is provided whereby laminate-type batteries can be manufactured much more inexpensively, in addition to the effects of the first to fifth aspects.
With this aspect, the porous film may be formed on the surface of both electrode plates or the porous film may be formed only on the surface of one electrode plate, when both the positive plate and negative plate have protrusions.
Incidentally, the porous film according to this aspect is preferably formed from thermoplastic polymers which are crystalline thermoplastic polymers with a melting point of 150xc2x0 C. or above and/or amorphous thermoplastic polymers with a glass transition point of 150xc2x0 C. or above. Such a porous film made of a thermoplastic polymer with a melting point or glass transition point of 150xc2x0 C. or above is resistant to deformation such as thermal contraction and melting, oxidative destruction, etc. up to high temperatures, particularly its melting point or glass transition point of 150xc2x0 C. or above. In other words, the porous film has superior heat resistance against high temperatures.
Since thermoplastic polymers have excellent plasticity at high temperatures, the separator can easily close pores when heated to high temperatures, particularly 150xc2x0 C. or above. The battery can therefore effectively exhibit a shutdown function with a temperature increase.
Thus, by using an electrode plate with this type of porous film integrally formed therein, even when the internal battery temperature reaches high temperatures exceeding 150xc2x0 C. it is possible to avoid melting and deformation such as heat contraction so that short-circuits between the positive pole and negative pole are avoided and heat generation by oxidative destruction may be avoided, so that an effect of further enhanced safety is provided for the battery.
(Seventh aspect)
The seventh aspect of the invention is characterized in that in the aforementioned sixth aspect, the porous film contains at least one type from among polypropylene, polybenzimidazole, polyimide, polyether imide, polyamidoimide, polyphenylene ether (polyphenylene oxide), polyallylate, polyacetal, polyamide, polyphenylene sulfide, polyethersulfone, polysulfone, polyether ketone, polyester resins, polyethylene naphthalate, ethylene-cycloolefin copolymers, polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer.
These heat resistant polymers are polymers with particularly high melting points or glass transition points even among heat resistant polymers with melting points or glass transition points of 150xc2x0 C. or above. Thus, by using an electrode plate with this type of porous film integrally formed therewith, it is possible to further enhance the safety of the battery without producing short-circuits even when the internal battery temperature exceeds 150xc2x0 C. These heat resistant polymers are also relatively easy to obtain and inexpensive. Consequently, this aspect allows easy and economical formation of porous films with very excellent heat resistance. In other words, electrode plates with such porous films integrally formed therewith can be easily and economically manufactured.
According to this aspect, therefore, an effect is provided whereby a battery with guaranteed safety can be obtained, in addition to the effects of the sixth aspect.
(Eighth aspect)
The eighth aspect of the invention is characterized in that in the aforementioned seventh aspect, the polyester resin is either or both polybutylene terephthalate and polyethylene terephthalate.
According to this aspect, fine pores are uniformly formed in the porous film so that the electrolytes easily pass through the porous film and the resistance of the porous film is lowered. As a result, the high current properties, etc. of the battery are enhanced.
(Ninth aspect)
The ninth aspect of the invention is characterized in that in any one of the aforementioned sixth to eight aspects, the porous film is provided with, in the direction of its thickness, a sponge-like interior and a surface having densely formed pores of a smaller size than in the interior. Since the interior is sponge-like in this porous film, it has large-sized pores and also a high porosity. The electrolytes can therefore move very easily and the permeability is thus very excellent.
On the other hand, the surface with densely formed pores of a smaller size than the interior can very effectively prevent deposition of dendrites in the negative plate. Because of the small size of the pores, those pores can be rapidly and fully closed under high temperature. The shutdown function can thus be even more effectively exhibited when the battery is at high temperatures. Also, since the surface has high density and strength, the mechanical strength of the porous film can be increased.
The porous film as described above has very excellent electrolyte permeability and can effectively exhibit a shutdown function even at high temperature. By using an electrode plate with a porous film of this type integrally formed therewith it is thus possible to further enhance the battery properties such as the load characteristics and output characteristics of the battery, while also more effectively preventing shorts and abnormal heat generation.
According to this aspect, therefore, an effect is provided whereby superior battery performance can be achieved and the safety of the battery can be enhanced, in addition to the effects of the sixth to eighth aspects.
(Tenth aspect)
The tenth aspect of the invention is characterized in that in any one of the aforementioned first to ninth aspects, either or both the positive plate and the negative plate consist of at least an active material and a binder, and the binder comprises a hydrophilic polymer material with hydrophilic groups which is crosslinked together via a crosslinking agent which undergoes a binding reaction with the hydrophilic groups.
According to this aspect, the binder which binds the active material comprises a hydrophilic polymer material which is strongly crosslinked with a crosslinking agent, and it therefore has high chemical stability and does not dissolve or react with the electrolyte solution. The electrolyte active material is therefore stably bound over long periods by the binder, thus allowing a satisfactory battery reaction to be reliably accomplished during that time.
According to this aspect, therefore, an effect is provided whereby superior battery performance can be achieved over long periods, in addition to the effects of the sixth to ninth aspects.
According to this aspect, the active material-containing mixture is prepared by combining the electrode active material, the hydrophilic polymer material with hydrophilic groups, the crosslinking agent with functional groups that can cause a hydrolysis reaction with the hydrophilic groups for binding, and a solvent, and the active material-containing mixture may be used to shape the electrode plate into the desired shape.
(Eleventh aspect)
The eleventh aspect of the invention is a process for manufacture of an electrode having a sheet-like electrode body and a protrusion that protrudes out beyond the edge of the electrode body, having an insulating porous film on the surface of the electrode body and having a short-circuit preventing film formed integrally with the porous film on the surface of at least the proximal section of the protrusion, characterized by comprising an electrode plate-forming step wherein an electrode plate is formed comprising the electrode body and the protrusion, a polymer mixture layer-forming step wherein a polymer mixture prepared by dissolving a polymer material in a good solvent therefor is adhered in the form of a layer onto the surface including the electrode plate and the proximal section of the protrusion, to form a polymer mixture layer on that surface, a polymer deposition step wherein the polymer mixture layer is exposed to a poor solvent for the polymer material to deposit the polymer material, and a drying step wherein the polymer material deposited from the polymer mixture layer is dried to obtain the short-circuit preventing layer and the porous film.
In the polymer mixture layer-forming step, a layer of a finely-dispersed polymer material is formed on the surface of the electrode plate. Since the good solvent and poor solvent for the polymer material in the polymer mixture layer are exchanged in the ongoing polymer deposition step, the solubility of the polymer material in the polymer mixture layer is lowered, resulting in deposition of the polymer material. In the subsequent drying step the solvent in the polymer mixture layer is evaporated to give a porous film with the function of a short-circuit preventing layer and separator.
In the electrode plate-forming step, the protrusion may be simply formed in addition to the electrode body, thus allowing easy and economical formation of an electrode plate with the electrode body and protrusion integrally formed, and avoiding the complex electrode plate-forming means required for publicly known electrode plate-forming methods. Simple apparatuses (equipment) may be used in all of the steps of the polymer mixture layer-forming step, the polymer deposition step and the drying step, thus greatly facilitating the operation. Each of the steps can therefore be carried out easily and economically.
This aspect may be used for any one of the aforementioned sixth to tenth aspects to easily and economically form an electrode provided with the short-circuit preventing layer and the porous film. An electrode manufactured according to this aspect may be used in the manufacture of a laminate-type battery according to any one of the aforementioned sixth to tenth aspects, to allow inexpensive manufacture of the laminate-type battery.
The short-circuit preventing layer and porous film may be formed separately when it is desired to change the materials of the short-circuit preventing layer and the porous film, but they are preferably formed simultaneously when they are to be made of the same material. By simultaneously forming the short-circuit preventing layer and the porous film it is possible to shorten the steps for formation of an electrode with a short-circuit preventing layer and porous film, thus improving the productivity of the electrode. The electrode can therefore be formed even more economically.
The polymer mixture layer-forming step, the polymer deposition step and the drying step according to this aspect may be carried out in the following manner.
[Polymer mixture layer-forming step]
The polymer material used may be a thermoplastic polymer which is selected from either or both crystalline thermoplastic polymers with a melting point of 150xc2x0 C. or above and/or amorphous thermoplastic polymers with a glass transition point of 150xc2x0 C. or above. There may be used one or more types from among polypropylene, polybenzimidazole, polyimide, polyether imide, polyamidoimide, polyphenylene ether (polyphenylene oxide), polyallylate, polyacetal, polyamide, polyphenylene sulfide, polyethersulfone, polysulfone, polyether ketone, polyester resins, polyethylene naphthalate, ethylene-cycloolefin copolymers, polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer. When a polyester resin is used, it is preferably either or both polybutylene terephthalate and polyethylene terephthalate.
There are no particular restrictions on the good solvent for the polymer material used in the polymer mixture layer-forming step, and it may be appropriately selected depending on the polymer material used. However, for most polymer materials including the polymer materials mentioned above, organic solvents may be used as the good solvents.
According to this aspect, particularly suitable good solvents are N-methyl-2-pyrrolidone (NMP), dimethylsulfoxide, dimethylsulfonamide, diglime, toluene, xylene, dimethylacetamide, dichloromethane, cyclohexane and cyclohexanone. These exemplary good solvents easily dissolve polymer materials, and can satisfactorily dissolve the aforementioned polymer materials in particular. Preparation of the polymer mixture is thereby facilitated.
In order to obtain a porous film with a uniform thickness, a surfactant, defoaming agent, surface modifier, etc. may be added to the polymer mixture. Such additives are preferably fluorine-based or silicon-based compounds that have low reactivity in the battery and exhibit an effect even when added in small amounts. The pore shape, size and distribution of the porous film can be appropriately adjusted by adding water, an alcohol, a glycol, a ketone or an alkane such as decane to the polymer solvent.
On the other hand, the conditions such as the concentration and temperature of the good solvent are preferably appropriately selected to allow satisfactory dispersion of the polymer material.
The polymer mixture may be in liquid form or in paste form. The method of adhering the polymer mixture as a layer on the surface of the electrode plate may also be appropriately selected depending on the form of the polymer mixture.
In the case of a paste-like polymer mixture, it may be applied by a coating method using a blade coater, roll coater, knife coater, die coater, comma coater, reverse roll coater or gravure coater, for adhesion of the polymer mixture as a layer onto the surface of the electrode plate. In these coating methods, the polymer solution is preferably a high viscosity solution to prevent the polymer solution from displacing the air in the pores of the electrode plate. For example, when a polyetherimide is used as the polymer material, a high viscosity polymer solution can be obtained by dissolving it in an amount of 10-30 wt % (solid concentration) with respect to the total polymer solution. A thickener or the like may also be added to increase the viscosity of the polymer solution.
In the case of a liquid polymer mixture, the polymer mixture may be sprayed onto the surface of the electrode plate, or the electrode plate may be immersed in the polymer mixture for adhesion. In the latter adhesion method, it is preferred to use a low viscosity polymer solution in order to improve the solution run-off when the electrode plate is pulled out from the polymer solution. For example, when a polyetherimide is used as the polymer material, a low viscosity polymer solution can be obtained by dissolving it in an amount of no greater than 10 wt % with respect to the total polymer solution.
A polymer film prepared by forming the polymer mixture into the form of a film can also be adhered (transferred) onto the surface of the electrode plate.
The thickness of the polymer mixture layer is preferably in the range of 20-100 xcexcm. If necessary, the density of the polymer mixture layer may be increased to the desired density by a method such as calender rolling, etc.
[Polymer deposition step]
In this step, the electrode plate coated with the polymer solution is immersed in a poor solvent for the polymer material (poorly-dissolving solution), or is exposed to a gas of the solvent in the vapor phase, to expose the polymer material to the poor solvent. For example, when a liquid form of the polymer mixture is used, the polymer mixture exposed to the poor solvent for the polymer material undergoes gelling, and the polymer material is deposited.
There are no particular restrictions on the poor solvent for the polymer material in this step, and it may be appropriately selected depending on the polymer material used and its good solvent. However, since it is preferred to use an organic solvent as the good solvent for most polymer materials including the polymer materials mentioned above, water, an alcohol, a ketone or an alkane such as decane is preferably used as the poor solvent.
In this step, the poor solvent used may be at least one from among water, alcohols and ketones as mentioned above, as well as sulfolane, xcex3-butyrolactone, formamide, nitrobenzene, propylene carbonate, ethylene carbonate, tricresyl phosphate and triphenyl phosphate. The poor solvents mentioned here have boiling points of 200xc2x0 C. and above, which are higher boiling points than the good solvents mentioned above. The phase separation of the polymer material is thus facilitated in the drying step.
On the other hand, the conditions including the concentration and temperature of the poor solvent are preferably selected as appropriate to facilitate exchange with the good solvent and allow satisfactory deposition of the polymer material.
[Drying step]
In this step, the solvent component of the polymer solution may be removed using a thermostatic chamber, a hot air drier, a vacuum drier, etc. Once the solvent component is removed from the polymer solution, the deposited polymer material becomes a porous film.
Incidentally, by appropriate selection of the conditions including the material, concentration and temperature of the poor solvent in the polymer deposition step, it is possible to rapidly exchange the good solvent and poor solvent on the surface in the direction of thickness of the polymer mixture layer. Rapid exchange of the good solvent and poor solvent on the surface in this manner causes polymer aggregation of the polymer material, and the polymer material is more finely deposited than toward the inside in the thickness direction. Meanwhile, although the exchange between the good solvent and the poor-dissolving solvent is slower toward the inside than in the surface, it proceeds relatively rapidly and uniformly, forming relatively thin fibrils and forming uniform gaps (which later become the pores) of a common size. This gives layers with different forms of deposition of the polymer material on the surface and toward the inside in the direction of thickness of the polymer mixture layer.
Next, drying of the polymer mixture layer in the drying step removes the solvent in the fibril gaps at the inside, uniformly forming pores of a common size and producing a sponge-like structure. On the surface, meanwhile, the solvent in the gaps of the finely deposited polymer aggregates is removed, producing a structure with dense pores of a smaller pore size than at the inside.
It is thus possible to obtain a porous film on the surface of the electrode plate which is provided with a sponge-like inside and a surface with dense pores of a smaller pore size than at the inside, in the direction of the film thickness.
This aspect, therefore, is used with the sixth to tenth aspects, and has an effect whereby the electrode provided with the porous film can be easily and economically formed. In addition, since it involves no drawing step such as required for film separators commonly provided as separate members, there is no thermal shrinkage due to increased battery temperature, and therefore short-circuiting between the positive plate and negative plate can be prevented and the safety enhanced.
(Twelfth aspect)
The twelfth aspect of the invention is characterized in that in the electrode plate-forming step of the eleventh aspect, an electrode active material, a hydrophilic polymer material with hydrophilic groups, a crosslinking agent that can cause a binding reaction with the hydrophilic groups for binding, and an aqueous solvent are combined to form a crosslinked polymer by crosslinking the hydrophilic polymer material with the crosslinking agent that has undergone binding reaction with the hydrophilic groups, and preparing an active material-containing mixture with the crosslinked polymer dispersed in the electrode active material, thereafter, the active material-containing mixture is applied to a collector and the electrode active materials is bound together by the crosslinked polymer to form the electrode plate.
According to this aspect, when the electrode active material, the hydrophilic polymer material, the crosslinking agent and the aqueous solvent are combined under prescribed conditions such as high temperature, etc., the hydrophilic polymer material is crosslinked by the crosslinking agent that has undergone binding reaction with the hydrophilic groups, forming a crosslinked polymer. The crosslinked polymer binds the electrode active material. Thus, the hydrophilic polymer material with hydrophilic groups and the crosslinking agent with functional groups that can bind with the hydrophilic groups by hydrolysis reaction become the binder of the electrode active material layer. The aqueous solvent used may be water or an aqueous solution, alcohol, etc.
According to this aspect the hydrophilic polymer material may be easily dispersed in the aqueous solvent. This aspect, therefore, allows an aqueous solvent to be used to easily prepare an active material-containing mixture in which the hydrophilic polymer material is satisfactorily dispersed in the electrode active material. When an electrode plate is formed using this active material-containing mixture, the hydrophilic polymer material is present as a satisfactory dispersion in the electrode active material, and therefore the electrode active materials have excellent bindability to each other. That is, since the electrode active materials are stably bound together by the hydrophilic polymer material, the electrode plate can reliably accomplish a satisfactory battery reaction.
Because aqueous solvents have lower flammability and volatility than organic solvents, they are very safe solvents. They are very safe for humans and are also easy to handle. It thus becomes possible to more easily accomplish the electrode plate-forming step using simpler apparatuses. The solvent collecting equipment is also less complex than the solvent collecting equipment for organic solvents. As a result, the cost of the electrode plate-forming step can be minimized.
Incidentally, methods of preparing active material-containing mixtures using aqueous solvents are known by combining rubber latexes with aqueous polymer materials such as cellulose, etc., such as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 4-342966.
However, conventional active material-containing mixtures have not had sufficiently high wettability with respect to collector sheets. It has therefore been difficult to form active material mixture layers of uniform thickness on the surfaces of collector sheets, since the active material-containing mixtures applied onto collector sheets form globular aggregates (or xe2x80x9cball upxe2x80x9d). There has also not been sufficiently high adhesion between the collector sheet and the electrode active material layer comprising the active material-containing mixture, and the often repeated use of such batteries has led to peeling of the electrode active material layer from the collector sheet, resulting in the problem of lower battery performance, including lower cycle properties.
According to this aspect, the wettability of the active material-containing mixture with respect to the collector sheet is higher than conventional active material-containing mixtures because the surface tension of the active material-containing mixture is lower due to the functional groups of the crosslinking agent. The active material-containing mixture can therefore be more easily applied onto the surface of the collector sheet, allowing the active material mixture layer to be easily formed to a uniform thickness.
Because the crosslinked polymer has excellent bindability not only to the electrode active material but also to the collector sheet, it can satisfactorily bind not only the active materials to each other but also the electrode active material and the collector sheet. The adhesion between the electrode active material layer and the collector sheet can therefore be improved. As a result, since the electrode active material layer does not easily peel from the collector sheet, the cycle properties of the battery can be improved, and reduction in battery performance with often repeated used of the battery can be easily prevented.
According to this eleventh aspect, when the porous film is integrally formed on the surface of the electrode plate formed in the electrode plate-forming step, the electrode plate is exposed to the good solvent for the polymer material in the polymer mixture layer-forming step, while the polymer mixture layer is exposed to a poor solvent for the polymer material in the subsequent polymer deposition step. In particular, when an organic solvent is used as the good solvent and water, an alcohol, a ketone or an alkane such as decane is used as the poor solvent, the polymer mixture layer is exposed to solvents of a vastly different nature, oily and aqueous. In the electrode plate-forming step it is therefore necessary to form an electrode plate which is non-soluble in both the good solvent and the poor solvent for the polymer material.
The crosslinked polymer formed by crosslinking the hydrophilic polymer material by the crosslinking agent that reacts with its hydrophilic groups possesses firm crosslinks, and it is therefore poorly soluble in both the well-dissolving and poor solvents for the polymer material, and is particularly insoluble in organic solvents and in water, alcohols, ketones, etc. With this aspect, therefore, in the electrode plate-forming step it is possible to form an electrode plate which is both organic solvent-resistant and water-resistant.
According to this aspect, an effect is provided whereby an electrode which allows a more satisfactory battery reaction to be reliably accomplished can be economically manufactured, the range of selection for the solutions used in the polymer coating step and polymer deposition step is wider and control over the quality of the porous film is made easier, in addition to the effect of the eleventh aspect.
With this aspect, a pH adjuster or the like may be added to the active material-containing mixture depending on the crosslinking agent used, to render the crosslinking agent more soluble in the active material mixture.
The content of the crosslinking agent in the active material-containing mixture may also be appropriately selected depending on the content of the hydrophilic polymer material, for satisfactory crosslinking of the hydrophilic polymer material. If the content of the crosslinking agent is too large with respect to the content of the electrode active material, it will coat the surface of the electrode active material and thus lower the battery reactivity of the electrode active material, resulting in lower electrode performance. The content of the crosslinking agent is therefore preferably selected as appropriate depending on the content of the electrode active material as well as the content of the hydrophilic polymer material, and more preferably it is also selected as appropriate depending on the particle shape and specific surface area of the electrode active material.
According to this aspect it is preferred to prepare the active material-containing mixture by either of the following two procedures.
(1) After dissolving the crosslinking agent in the aqueous solvent to the prescribed concentration, a prescribed amount of the electrode active material is added and the solution is stirred. A prescribed amount of the hydrophilic polymer material is then added to the solution which is thoroughly kneaded to obtain a paste-like active material-containing mixture.
(2) A prescribed amount of the electrode active material is added to and thoroughly stirred with the aqueous solvent. A prescribed amount of the hydrophilic polymer material is then added to the solution which is thoroughly kneaded to obtain a paste-like active material-containing mixture. Finally, the crosslinking agent is mixed with the active material-containing mixture. When mixing the crosslinking agent, if the crosslinking agent is a solid (powder) it is preferably dissolved first in an aqueous solvent and then combined with the active material-containing mixture. On the other hand, if the crosslinking agent is liquid, it is preferably mixed directly with the stock solution to prevent drastic changes in the viscosity of the active material-containing mixture.
(Thirteenth aspect)
The thirteenth aspect of the invention is characterized in that, in the electrode plate-forming step of the aforementioned twelfth aspect, the hydrophilic groups are hydroxyl groups. The hydrophilic polymer material crosslinked by binding groups produced by reaction of hydroxyl groups can form a very strong crosslinked structure because of their strong polarity compared to other hydrophilic group-based binding groups. With this aspect, then, it is possible to further improve the non-solubility of the binder with respect to the good solvent and poor solvent, and to form an electrode plate which is both organic solvent-resistant and water-resistant.
According to this aspect, an effect is provided whereby the range of selection for both the good solvent used in the polymer coating step and the poor solvent used in the polymer deposition step is wider and control over the quality of the short-circuit preventing layer and the porous film is made easier, in addition to the effect of the twelfth aspect.
With this aspect, the crosslinking agent used is one with functional groups that can bind by a binding reaction with the hydroxyl groups.
(Fourteenth aspect)
The fourteenth aspect of the invention is characterized in that in the aforementioned thirteenth aspect, the hydrophilic polymer material is at least one from among carboxymethyl cellulose, methyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid salts and polyethylene oxide.
These hydrophilic polymer materials all have hydroxyl groups and are also relatively easy to obtain and inexpensive. With this aspect, then, it is possible to easily and economically form an electrode plate with excellent non-solubility in the good solvent and the poor solvent.
All of these hydrophilic polymer materials can provide excellent organic solvent resistance, and can therefore render the electrode plate very highly insoluble in organic solvents. Thus, when an organic solvent is used as the good solvent in the polymer mixture layer-forming step, very high insolubility is afforded with respect to the good solvent.
According to this aspect, therefore, an effect is provided whereby the range of selection for the good solvent used in the polymer coating step is wider and control over the quality of the short-circuit preventing layer and the porous film is made even easier, while the electrode provided with the short-circuit preventing layer and the porous film can be manufactured more easily and economically, in addition to the effect of the thirteenth aspect.
With this aspect, the carboxymethyl cellulose, methyl cellulose, ethyl cellulose, polyvinyl alcohol and polyethylene oxide may also be used in the form of salts, such as sodium salts.
(Fifteenth aspect)
The fifteenth aspect of the invention is characterized in that in any one of the aforementioned twelfth to fourteenth aspects, the crosslinking agent has hydrophilic groups. Since the crosslinking agent used for this aspect has hydrophilic groups it can be easily dissolved in aqueous solvents. It can therefore be thoroughly distributed in the active material-containing mixture to cause effective crosslinking of the hydrophilic polymer material.
For this aspect it is particularly preferred for the crosslinking agent to be one with vinyl, glycidoxy, amino, diamido or ureido functional groups. These crosslinking agents have high affinity for water or aqueous solutions at pH 7. Since they can therefore dissolve easily in water or aqueous solutions at pH 7, it becomes unnecessary to add a pH adjuster or the like to the active material-containing mixture. As a result, the cost for preparation of the active material-containing mixture can be lowered.
Amino and diamido functional groups are the preferred functional groups among the functional groups mentioned above. These functional groups all have excellent resistance to reduction and therefore even when the electrode is used at a strong reduction potential, the crosslinking of the hydrophilic polymer material is maintained. Reduced binding strength between the electrode active materials and between the electrode active material in the electrode and the collector sheet is therefore avoided.
(Sixteenth aspect)
The sixteenth aspect of the invention is characterized in that, in any one of the aforementioned twelfth to fifteenth aspects, the crosslinking agent is at least one from among silane coupling agents, titanium coupling agents, urea formalin resins, methylol-melamine resins, glyoxal and tannic acid.
Since these crosslinking agents have functional groups with excellent reactivity for the hydroxyl groups of the hydrophilic polymer material, they can effectively cause crosslinking of the hydrophilic polymer material. It thus becomes possible to more easily and reliably crosslink the hydrophilic polymer material. That is, the crosslinking density of the hydrophilic polymer material is highest and the crosslinks of the hydrophilic polymer material can be stronger, even within the aforementioned fourteenth aspect. As a result it is possible to improve the non-solubility of the electrode plate in the good solvent and the poor solvent.
These crosslinking agents are also relatively easy to obtain and inexpensive. Consequently it is possible to easily and economically form an electrode plate with excellent non-solubility with respect to the good solvent and the poorly dissolving solvent.
According to this aspect, therefore, an effect is provided whereby the range of selection for both the good solvent used in the polymer coating step and the poor solvent used in the polymer deposition step is wider and control over the quality of the short-circuit preventing layer and the porous film is made even easier, while the electrode provided with the short-circuit preventing layer and the porous film can be manufactured more easily and economically, in addition to the effects of the twelfth to fourteenth aspects.
Incidentally, according to this aspect it is preferred to use a silane coupling agent, among the crosslinking agents mentioned above. Silane coupling agents have alkoxy groups, and the alkoxy groups hydrolyze in the active material-containing mixture to produce silanol groups. Since silanol groups readily undergo binding reaction with hydroxyl groups, it is possible to effectively crosslink hydrophilic polymers with hydroxyl groups. Very strong crosslinks can thus be created in the hydrophilic polymer material with hydroxyl groups. The silanol groups can also improve the wettability of the active material-containing mixture. It thereby becomes easier to coat the active material-containing mixture onto the surface of the collector sheet, thus facilitating formation of an active material mixture layer of uniform thickness.
Preferred among silane coupling agents are those with vinyl, glycidoxy, amino, diamido and ureido functional groups, and more preferred are those with amino and diamido functional groups. As an example of such a silane coupling agent there may be mentioned xcex3-aminopropyltriethoxysilane. Such a silane coupling agent will be readily dispersible in the active material-containing mixture and can effectively crosslink the hydrophilic polymer material.
The content of the silane coupling agent in the active material-containing mixture is preferably selected as appropriate depending on the amounts of the hydrophilic polymer material and the electrode active material, as well as on the particle shape and specific surface area of the electrode active material, for the same reasons given for limiting the content of the crosslinking agent as explained for the aforementioned eleventh aspect. It is particularly preferred for the silane coupling agent content to be no greater than 4 wt % with respect to 100 wt % as the total active material-containing mixture. If the silane coupling agent content exceeds 4 wt %, it will excessively coat the surface of the electrode active material and thus lower the battery reactivity of the electrode active material.
(Seventeenth aspect)
The seventeenth aspect of the invention is characterized in that, in any one of the aforementioned twelfth to sixteenth aspects, the crosslinking agent contained in the active material-containing mixture has functional groups that undergo binding reaction with the hydrophilic groups in the hydrophilic polymer material, in the same number as or greater number than the hydrophilic groups.
With this aspect, the crosslinking agent can undergo crosslinking reaction with all or almost all of the hydrophilic groups in the hydrophilic molecule material. That is, it is possible to achieve particularly high crosslinking density of the hydrophilic polymer material and particularly strong crosslinks in the hydrophilic polymer material, for the aforementioned twelfth to sixteenth aspects. As a result, the non-solubility of the electrode plate in the good solvent and poor solvent can be especially improved.
According to this aspect, therefore, an effect is provided whereby the range of selection for both the good solvent used in the polymer coating step and the poor solvent used in the polymer deposition step is wider and control over the quality of the short-circuit preventing layer and the porous film is made even easier, in addition to the effects of the twelfth to sixteenth aspects.
For example, if a silane coupling agent: H2NC3H6Si(OC2H5)3 is added to carboxymethyl cellulose sodium salt: C6H7O2(OH)2OCH2OONa, where the molecular weight of the carboxymethyl cellulose sodium salt is 242, the number of hydroxyl groups dissolved in water is 3, the molecular weight of the silane coupling agent is 221 and the number of hydrolyzing groups is 3, it is preferred for the silane coupling agent to be added at least to (221/3)/(242)/3)=0.91, with respect to the carboxymethyl cellulose weight.
(Eighteenth aspect)
The eighteenth aspect of the invention is characterized in that, in any one of the aforementioned eleventh to seventeenth aspects, the polymer mixture comprises a mutual mixture of the polymer material and a salt.
With this aspect, the salt is extracted into the poor solvent in the polymer deposition step, forming pores that allow the electrolytes to pass through the porous film. It is thereby possible to easily form a porous film with excellent electrolyte permeability on the surface of the electrode plate. The pore size of the porous film can also be easily controlled by merely selecting the amount of salt added.
According to this aspect, therefore, an effect is provided whereby an electrode can be easily formed having a porous film with excellent electrolyte permeability, in addition to the effects of the eleventh to seventeenth aspects. By using an electrode manufactured according to this aspect in a laminate-type battery, high output characteristics can be easily achieved and superior battery performance can be easily exhibited.
(Nineteenth aspect)
The nineteenth aspect of the invention is characterized in that, in the aforementioned eighteenth aspect, the salt is at least one from among lithium chloride, lithium nitrate, lithium iodide, lithium tetrafluoroborate, lithium bistrifluoromethylsulfonylimide and lithium hexafluoroarsenate. Because these lithium salts have excellent solubility in the poor solvent, the salts are extracted more readily into the poor solvent and pores that allow permeation of the electrolytes can be formed with particular ease in the porous film. These lithium salts are also relatively easy to obtain and inexpensive. It thus becomes possible to easily and economically form porous films with excellent electrolyte permeability.
According to this aspect, therefore, an effect is provided whereby an electrode can be inexpensively formed having a porous film with excellent electrolyte permeability, in addition to the effect of the eighteenth aspect.
Incidentally, when an electrode manufactured according to this aspect is used in a lithium secondary battery, even if such lithium salts remain in the porous film it is possible to prevent reaction between the remaining lithium salt and the lithium salt contained in the electrolyte solution. Even if the salts contained in the porous film dissolve out into the electrolyte solution, there is no effect on the electrode reaction so that excellent battery performance can be maintained.
(Twentieth aspect)
The twentieth aspect of the invention is characterized in that, in either of the aforementioned eighteenth and nineteenth aspects, the polymer mixture contains the salt at 5 parts by weight or greater with respect to 100 parts by weight of the polymer material. If the concentration of the lithium salt in the polymer mixture is under 5 parts by weight the size of the pores formed in the porous film will be too small, making it difficult to achieve excellent electrolyte permeability. This will impede the excellent battery performance, such as high output characteristics, for the laminate-type battery. Since the lithium salt concentration is controlled to 5 parts by weight or greater according to this aspect, it is possible to more easily form pores in the porous film to a size sufficient to allow permeation of the electrolytes. A porous film with excellent electrolyte permeability can thus be formed more easily.
According to this aspect, therefore, an effect is provided whereby an electrode with excellent electrolyte permeability can be easily manufactured, in addition to the effects of the aforementioned eighteenth and nineteenth aspects.
On the other hand, it is also preferable for the concentration of the lithium salt in the polymer mixture to be no greater than 20 parts by weight with respect to 100 parts by weight of the polymer material. If the lithium salt concentration exceeds 20 parts by weight the pores in the porous film will be too large, making it difficult to achieve a porous film with excellent performance, such as a shutdown function. Since the lithium salt concentration is controlled to no greater than 20 parts by weight according to this aspect, it is possible to easily form pores in the porous film to a size sufficient to provide a shutdown function. A porous film with excellent performance including a shutdown function can thus be formed more easily.
According to this aspect, therefore, an effect is provided whereby an electrode having a porous film with excellent performance including a shutdown function can be easily manufactured, in addition to the effects of the aforementioned eighteenth and nineteenth aspects.